1. Field of the Invention
The present invention relates to a method of manufacturing cured (i.e., crosslinked) ethylene acrylate elastomer or polyacrylate elastomer. More specifically, the present invention relates to the use of an aqueous solution of hexamethylene diamine (HMDA) as a curative agent for ethylene/alkyl acrylate copolymer and poly(alkyl acrylate) copolymer containing a mono alkyl ester cure-site termonomer.
2. Description of the Related Art
It is generally known in the art to cure a terpolymer of ethylene, an alkyl acrylate, and a mono alkyl ester cure-site monomer with hexamethylene diamine carbamate (HMDAC) to produce vulcanized elastomer. The resulting ethylene acrylate elastomers exhibit a combination of mechanical toughness, low brittle point, low oil swell, and a high level of heat aging resistance that is particularly conducive to certain commercial automotive applications. For example, U.S. Pat. No. 3,904,588 describes and claims the curing of random copolymer derived from the polymerization of ethylene, an alkyl acrylate (e.g., methyl and ethyl acrylate) and a monoester of maleic acid (e.g., methyl, ethyl, and propyl 1,4-butene-dioic acid). This reference discloses and claims the use of 1.5 parts of HMDAC per 100 parts by weight of the terpolymer containing the mono ester cure-site monomer along with other ingredients being blended on a roll mill followed by a press-curing step for 30 minutes at 180° C. at a total pressure of about 40,000 psig. The reference teaches that hexamethylene diamine can be used as the vulcanizing agent but does not teach the use of an aqueous solution of HMDA.
In a companion U.S. Pat. No. 3,883,472 filed on the same day, an elastomeric composition having good scorch resistance is taught involving an acrylic ester/butenedioic acid monoester dipolymer or an ethylene/acrylic ester/butenedioic acid monoester terpolymer which is crosslinked with a vulcanizing agent and at least one vulcanization accelerator. Hexamethylene diamine carbamate, tetramethylenepentamine (TEPA), and hexamethylene diamine are employed as the curing agent during vulcanization. However, there is again no disclosure of the use of an aqueous solution of HMDA. Similarly, it is also generally known that polyacrylate elastomers typically involving two or more alkyl acrylates (e.g., ethyl acrylate, butyl acrylate and 2-methoxyethyl acrylate) copolymerized with a small amount of butene dioic acid monoalkyl ester cure-site termonomer can be crosslinked during curing by the use of various diamines such as HMDA and HMDAC, see for example Japanese patent application JP 2000-44757.
In U.S. Pat. No. 4,412,043 an elastomeric composition derived from an ethylene/methyl or ethyl (meth)acrylate copolymer having a third cure-site involving 4-(dialkylamino)-4-oxo-2-butenoic acid termonomer is taught by employing a diamine curing agent. Again HMDA is identified as an alternative to HMDAC but no disclosure of the use of an aqueous solution of HMDA is present.
In U.S. Pat. No. 4,026,851, the use of an aqueous 88% hexamethylene diamine solution is employed as a curing agent for curing a polyacrylate, ethylene/acrylate copolymer or ethylene/acrylate/methacrylate terpolymer wherein no monoester cure-site monomer is present.
The problem with the above methodology for curing is that HMDAC is a relatively expensive crosslinking agent and even though it is a minor constituent it represents a significant cost and HMDA is relatively corrosive and difficult to handle with a melt/freeze point of about 40° C. As such, an alternate methodology and low cost alternative crosslinking agent would be potentially advantageous.